Content sources as data processing pipelines

_Note: This issue is WIP and needs to be expanded._

## Notes

- Differentiate between **input data** and **output data**
- Input data validation could be done via Zod
- Question: How do define the output schema? (based on the input schema)
- Other schema formats: #163
- #250
- Also see [this comment](https://github.com/contentlayerdev/contentlayer/issues/214#issuecomment-1170856380)
- This would also unlock #84
- Make it easy to let the user define what they want to access (e.g. pre-rendered HTML vs raw MD source)

### Markdown/MDX processing pipeline

- This could allow for a smoother unified/remark/rehype content processing integration
- #137

## Related

- #120
- #163
- #106
- #84
- #160
- #91
- #275
- #86
- #71
- #148
- #24
- #149
- #26
- #216
- #225
- #325
- [Private Notion doc](https://www.notion.so/schickling/Build-pipeline-explorations-8214b31765bb48b9814d4dfaad23dfe8)

先从此 issue 的说明以及 #163、#214 和 #137 中链接的讨论开始，然后查看相关 issue 和 build-pipeline 的探索。定义输入和输出数据模型、schema 方案、可选择的 raw 或 rendered 内容，以及 Markdown/MDX 的处理行为；当 pipeline 范围和验收标准完成文档化后，即视为完成。

由索引模型根据 Issue 内容生成。